Microbial causes of ventilator-associated pneumonia.

Aspiration of microorganisms colonizing the oropharynx is the main route of bacterial entry to lower airways in mechanically ventilated patients. Examination of the microbial flora involved in ventilator-associated pneumonia shows that only few species, among the many oropharynx microorganisms, are responsible for the majority of lower respiratory tract colonizations and infections in intubated patients. Underlying disease, length of intubation, and type and duration of prior antibiotic therapy are the most important factors related with the causative flora of respiratory infections in these patients. Except in certain populations (eg, chronic obstructive pulmonary disease [COPD] patients who may be colonized by Pseudomonas aeruginosa), methicillin-sensitive Staphylococcus aureus, Streptococcus pneumoniae, and unencapsulated Hemophilus influenzae are the predominant respiratory pathogens within the first week of intubation in critically ill patients. These microorganisms are subsequently replaced by multiresistant flora, such as Pseudomonas aeruginosa, methicillin-resistant staphylococci or Acinetobacter baumanii. This change of flora takes place as a consequence of prior antibiotic therapy among other factors. Fungi have to be taken in account particularly in the presence of severe immunodepression. All of these multiresistant pathogens (particularly P aeruginosa) are responsible for most of the deaths directly related to pneumonia; therefore, the early recognition of causative agents and appropriate antibiotic therapy are of great importance determining outcome. This strategy represents the most efficient approach to managing patients with ventilator-associated pneumonia.